1. Field of the Invention
The present invention relates to a zoom lens suitable for use as an imaging optical system in an image pickup apparatus such as a video camera, a digital still camera, a broadcast camera, or a silver-halide camera.
2. Description of the Related Art
In recent years, an imaging optical system used in an image pickup apparatus has been requested to have a high zoom ratio and a compact size, and is particularly required to be formed by a zoom lens that allows thinning of a camera. A so-called retractable type zoom lens is known in which lens units are stored in a camera housing in a non-image-taking state by reducing gaps between the lens units to gaps different from those in an image taking state in order to achieve a smaller size of the camera and a higher zoom ratio of the zoom lens. Also, a so-called bent type zoom lens is known in which a reflecting prism for bending the optical axis of an imaging optical system 90 degrees is provided in the optical path so as to reduce the thickness of the camera. Examples of bent-type lenses are disclosed in U.S. Pat. No. 6,333,823 and U.S. Patent Application Publication No. 2007/0014031 A1. Moreover, as a combination of the above-described types of zoom lenses, U.S. Patent Application Publication No. 2007/0091200 A1 discloses a so-called bent and retractable type zoom lens in which a reflecting prism moves in a non-image-taking state so that lens units on an object side of the reflecting prism are stored in a space formed by the movement of the reflecting prism.
When a retractable type zoom lens includes a reflecting prism for bending the optical path of an imaging optical system, a high zoom ratio can be easily obtained, and a camera to which the zoom lens is applied can be thinned easily. However, to obtain these advantages, it is important to properly set the lens configuration of the zoom lens, the structure of the reflecting prism, and the layout in the optical path. For example, it is important to properly set the number of lens units, the layout of refractive powers of the lens units, moving conditions of the lens units for zooming, the material and length in the optical axis direction of the reflecting prism, and the position of the reflecting prism in the optical path. Unless these structures are properly set, it is difficult to obtain the above-described advantages. In the zoom lens disclosed in U.S. Patent Application Publication No. 2007/0091200 A1, since the reflecting prism is provided in the second lens unit, the second lens is incapable of moving for zooming. For this reason, the moving amount of the first lens unit for zooming increases, and it is therefore difficult to reduce the thickness of the camera. Further, since the moving amount of the first lens unit is large, it is difficult to reduce the thickness of the camera by moving the reflecting prism and putting the lens units, which are located on the object side of the reflecting prism, into the camera body for storage.